Video signal processing systems (e.g. a color video camera which performs a raster type scan of its image-sensor area) typically employ a non-light receiving or optically shielded region which, when scanned, provides a `black` or `dark` level relative to which the output of the image receiving area of the optical sensor is defined (clamped). FIG. 1 shows, in simplified form, the opto-electronic response across a linear traverse of an optical sensor array, the response characteristic containing a varying signal region 10, and a `dark` region 12 whereat the sensor elements are shielded from incident light. During each successive blanking portion (the return sweep interval) of the video raster scan signal, the value of the dark region is measured, thereby periodically updating or restoring the D.C. level.
In a solid state imaging system, the behavioral characteristics of the individual devices may dictate that the output of each pixel be sampled, rather than read out via a continuous scan. For example, where the imaging system is comprised of a matrix of charge coupled devices, the light response information component is contained in a charge `bucket` portion, intermediate reset pulse and reset level portions of its output characteristic. Consequently, as the matrix is scanned, each CCD pixel is sampled only during this charge bucket portion. Unfortunately, the clocking signal that controls the sampling interval introduces artifacts in the output signal, shown diagrammatically in FIG. 2 as switching spikes or pulses 14. These artifacts represent correlated noise that injects an offset error into the black level.
More particularly, as illustrated in FIGS. 3 and 4, which show the effect of varying the gain of a pair of successive dark pixel samples 21 and 22, if a variable gain adjustment of the video signal is employed, it will also vary the magnitude of the sampling artifacts, so that the average value of the black signal, as measured by an associated sample and hold circuit, will vary from its true value, causing the information signal to be erroneously referenced. As shown in FIG. 4, which shows the effect of a gain increase from the signal levels of FIG. 3, the black level drops to a lower level 26' relative to its average level 24. Since the information signal is typically digitized for subsequent processing, what will be sampled and quantized by a downstream analog-to-digital converter (ADC) will yield an output code representative of a level other than a true pixel value.